Fabricating a TPS Driver

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Written by Randy Fox (randysgym)

Thursday, 27 August 2009

Page 1 of 10

Introduction

All Road Stars built before the fuel injected, 2008 models vary their ignition timing via a small sensor attached to the carburetor. This sensor is called the Throttle Position Sensor -- or TPS. Theoretically, the more you twist your right wrist, the more demand is placed on the engine, and the less the ignition timing should be advanced. In real life riding this theory has a few holes, but in my experience the TPS still out-performs other available methods for the Road Star carburetors, including V.O.E.S. and M.A.P. (See the Installing a V.O.E.S. article in this website for more information on VOES and MAP). The TPS offers the potential for better torque at all speeds, better fuel economy, and longer engine life.

Unfortunately, few aftermarket carbs include a way to connect the TPS. Carburetors built for high performance (racing) applications have no need for variable ignition timing since it is assumed that the engine demands will always be maximum, and therefore the mechanic just sets the timing to a fixed point.

For many owners however, the temptation to upgrade from the stock carb to achieve more power, quicker response, more reliability, and easier tune-ability outweighs the benefits that the TPS provides, and so the TPS comes out with the stock carb. In an effort to have both a better carburetor and proper ignition-advance, a few ingenious owners have fabricated custom brackets for certain, popular, aftermarket carbs to adapt their stock TPS. However, most have simply ignored the engine's needs for variable ignition timing in their thirst for more peak horsepower.

My design has one moving part: an axle. The axle is rotated via an integrated pulley that accepts the throttle pull cable -- which has been slightly shortened. The end of the little axle is fastened directly into the TPS, which moves the same way as it did in the stock carb. Another, tandem pulley, integrated into my axle, accepts a very short cable that, in turn, actuates my aftermarket carb.